I've been doing editing in FCP X on my rMBP but the 512GB SSD in it is only enough for one project at a time @ prores 422. And when I start school again next year I'll probably run into issues because of that. I'll be editing 1080p prores 422 with my files loaded on the external HDD so any advice on a portable external HDD that would suit my needs. I'll probably need 1TB on the go.

I personally use a 2TB My PassPort Studio Drive as it has a FireWire800 interface. USB2 would be too slow.
Ive alway used this as my scratch disk for iMovie with no problems whatsoever.
I now also use it as my scratch disk for FCPX and have had no issues.

My video is also 1080p using ProRes422.

USB3 drives are cheaper then Firewire though and are faster, however I have read that FireWire and Thunderbolt drives are best for video editing due to the way they stream data at a steady rate, rather then USB, which sends data in bursts in packets etc.

Not sure if this is correct, as I wouldnt mind using USB3 either, but for now my FireWire800 drives are perfectly fine with FCPX.